1. Field of the Invention
The disclosure relates to a method and apparatus for providing high purity stable 2H silicon carbide. More specifically, the disclosure relates to a method and apparatus for growing silicon carbide at temperatures well below melting temperature of silicon.
2. Description of Related Art
Silicon Carbide (“SiC”) has become an important wide bandgap semiconductor material because of its excellent properties for high power microwave devices. SiC now competes with GaAs and pure silicon in terms of gain, power output and efficiency at X-band. SiC promises even better performance at the higher frequencies (i.e., Ka and Ku-bands). Broadband power RF transmitters are needed with high efficiency, high linearity and low noise for transceiver modules. Silicon carbide crystallizes in more than 200 different modifications or polytypes. The most important polytypes are the so-called 2C, 4H and 6H, where “C” denotes cubic and “H” denotes hexagonal crystalline shape. Commercially available 4H- and 6H-SiC are mixture of cubic and hexagonal SiC crystals. As used herein, the terms 2H crystalline SiC and hexagonal SiC are interchangeable since 2H-SiC is pure hexagonal crystals.
The material attributes of SiC makes it particularly desirable for constructing communication and power devices. Such attributes include a relatively wide bandgap, a high thermal conductivity, high breakdown field strength and a high electron saturation velocity. SiC is commonly used in the bipolar junction transistors (“BJT”) and the Schottky diodes. BJTs are defined by two back-to-back p-n junctions formed in a semiconductor material. In operation, current enters a region of the of semiconductor material adjacent one of the p-n junctions called the emitter. Current exists the device from a region of the material adjacent the other p-n junction, called the collector. The collector and the emitter have the same conductivity type. A thin layer of semiconductor material, called the base, is positioned between the collector and the emitter. The base has opposite conductivity to the conductor and the emitter. High purity 2H-SiC has been found to be advantageous for use in bipolar junction transistors.
Similarly, diodes made of 4H SiC have been known to rapidly degrade and exhibit a growth of stacking faults under a forward bias application. In contrast, diodes made of 6H-SiC have been substantially less likely to degrade under a similar forward bias. Thus, high purity 6H-SiC have been advantageous over the 4H-SiC for such applications.
2H-SiC defines a novel class of SiC with wide bandgap material for broad applications. 2H-SiC can be used to make superior SiC devices in comparison to those fabricated from 4H- and 6H-SiC polytypes. This is due to the superior material properties of 2H in comparison with 4H and 6H. Due to difficulties in the growth of 2H, it is not offered commercially.
Conventional static induction transistors (SITs), Schottky diodes and metal semiconductor field-effect transistors (“MESFETs”) use 4H-SiC as it provides high bandwidth and high thermal conductivity. The SITs have been used for high frequency and high power applications. Since 4H-SiC has a large breakdown voltage and high thermal conductivity, it is used for making transistors with extremely high current density at high voltages. For other classes of MESFET devices, mobility and bandgap are important which makes 2H-SiC more suitable. Table 1 tabulates the physical and electrical characteristics of various SiC films.
TABLE 1Mobility @Sat. Elec. Poly-1 × 1017 cm−3 VelocitytypeBandgap (eV)(cm2/Vs)(Cm/S)2H-SiC3.357402.6 × 1074H-SiC3.285601.9 × 1076H-SiC3.083304.6 × 107
Another important aspect of 2H-SiC is its crystal structure and its advantages in relation to defect formation. 6H- and 4H-SiC are 33% and 50% cubic, respectively. In contrast, 2H-SiC is 100% hexagonal and 0% cubic. With 2H being a purely hexagonal structure, this could minimize the formation of stacking faults, edge, screw and mixed defects. Such defects are deleterious to device performance.
The conventional processes for preparing SiC result in polytypes of SiC, including 4H- and 6H-SiC. The conventional methods also require high-temperature processing of silicon carbide which creates defects in the final product, particularly 2H-SiC. Thus, conventional processes are not suitable for large scale manufacturing of 2H-SiC and there is a need for a method and apparatus for providing high mobility SiC for use in power devices in which 2H-SiC comprises more than 90%, preferably more than 98% and even more preferably about 100% of the SiC composition.